Velocity microphone with ribbon supported along its edges



June 6, 1950 w. D. CRAGG 2,510,079

VELOCITY MICROPHONE WITH RIBBON SUPPORIEO ALONG ITS EOGES 2 Sheena-sheet 1 Filed Dec. 19, 1946 Inventor June 6, 1950 w. D. cRAGG VELOCITY MICROPHONE WITH RIBBON SUPPORTED ALONG ITS EDGES 2 SheetsfSheet 2 Filed Dec. 19, 1946 n64v l e 6 Y nuenlor Harney Patented June 6, 1950 VELOCITY MICROPHONE wrrn RIBBON; sUPPoR'rED ALONG Irs VEnoes WilliamDonald'Cragg, LondonEngland, assigilor. a

to International- Standard Electric Corporatimn,l l NewnYork,pN.vY. a corporation of Delaware ApplcationDecember 19, 1946;Serial No. 717,261.

In YGreat Britain December. 11, 19451.

sectioni, Public Law ssa- August s, 19.46 Patent expires December 11., 19.6.5.

4 Claims.-

This invention relates to velocity (or pressure gradient) microphones.

The object of thisinvention isgto avoid Ycertain constructional-lfeaturesinherent inv microphonesof this ,type atpr'esent in use which render it'impossibletoA use the-magnetic i'leld efficiently, and result in dimensions far greater. thanY are ideally necessary.;

Such microphones are usually oi the ribbon type in which the diaphragm, is 'an aluminum ribbon ofthe order of 2" long X 0.27; Wide x 0.000,2"

thick clampedv at eachrend'and suspendedrin a transverse magnetic field;

The natural resonance `frequencyaof this ribbon n must Vbe near thelowcst` audible freduencm i. e.

20 to 30 C.'P.'S.and, inf orderpto achieve this, the

stiffness of the suspensionA must be Avery low ivl;1ich`. n

means that themean static position, of theuribbon is not rigidlyldetermined. YIn order that the rib-w.

bon Shell moveV in uniform. magnetic. eldwhatf.

ever its Stable position.. thedeothof. .theA airesao must be about 0.1 Whereas themovement oi the; ribbon due 4to a soundjield; is not greater than- According ,tothe present. invention. the ribbon of a velocity. meropheneis Suooortedin .sueh .a manner that its'static'position in theairgapoi Y the system .isolosely delinea by. supporting. the

ribbon. along theulllength of .its edgiesinstead of..

only at its.ends;.thusa11owingtbe- .volume ofthe magnet system 4to.be redllid.. without 'affecting use-` in the microphone shown inigEigures l and'zgg Figure 4 a cross sectionakplan view;of `a magnet-l-'system with a diterentform `of; -diae` phrasmingi Figure 5 shows. the.,.magnetM system and diaphragm of Figure 4 in elevation, While Figure .6 is a View simila`r..to.Fig. 4 showing a modified-form. of .the diaphragm oiiiigure 4. n

The invention was irsttried out experimentally with a at gold leaf ribbon 4, Figures 1 and 2 about 5.10-6 inches thick which was mounted slackly between and attached to the pole-pieces 2, 3 carried by a semi-circular magnet I. The rib- 2 .i bon was longerthan the pole pieces and wasn also Y fixed at its end to which theelectricalconnec-,U tions were made. l

Thetension acrossthe ribbon varied vfrom point te point aloneits1ensibeeibat-the naturelles: onance frequency ofg dirierentvsections of `the ribbonwere. spread over e .large ireqbeneyrense? in Ye.onseeluerlee,.there was. unmarked base reso.; nente.. A...f.eir1.yuniff rm response obtained. fromA n200 6000,. C. Pt, S.,Vthehigh balsswcut off, was. due-t0 theabsenee of eorrugatione inthe. ribbon.

If such a ribbon .hai/inse length. tenftimes. its. Width were provided with longitudinal corrugar., tionselong. each. ,edgebarins a radius :onerienth that used .forV lateral eorrusations ..in.the..usu.a1 endfmounted ribbons.. the .thickness of the .foi1..be... ing of the. order a of Vonehundredth ,ofv the usual thickness of ribbons, the bass cut ofi Wouldzbe the regionofxOC. P.

Since va ribbon microphoneis a masscontrolled.. devicethere is iorribbonspf.identical duinen-` sions a loss of. about 20 db, in using goldl instead,V 0i. aluminium, the .specie gravity. of. sold.. being 19.5 Ycompared with V2.5 for,aluminium.V

This .construction ,is capableoi producing. a ,l microphone with asensitivity oi 90 db... below 1 volt/dyne/sq.- .cm. for, a gold rilobnmfl .,25,..0hm s resistance.

Thismethodof construction .is perhaps, more of academic thanpractical ,.interest, as,.su cha` thin ribbon mai7 not be robust enough to stand .therev maltreatment which microphones xusually.,` receive.-

If5however `the ribbon madeoia thermo;V plasticma-terial such as .celluloe acetate, ornylon.... ofthe order of v10ft-10T@ inchesthichhavng a Youngs -modulusmuch lower even-.than a tl,1nnin ium or light alloy; longitudinally `corrugated at the; edges,VV and provided with.. an aluminiumkor. light alloy armaturerv 6 .of .Figure g3 .ofaboutythe samefthickness, the .natural resonance ofthe-dias: e pbraem een. easily be .kept .in .the region .0f-...50... C. P.S..

Bwslightadjustment .of the spacing of- .the pole -;-pi eees .aiterf-e-eseembling...theribbons, the stiffness .can bemadeprogressively variable along.; thefleneibofftheribbonfand-.oonseouent1y;.the. ribbon can be made to have different resonant?,

'frequeneiesalons .diferent sections-of ,ts...lengi.h

same ratio, the dimensions of the gap being, in a preferred embodiment, 0.156 inch wide x 0.0625 inch deep (pole-tip width).

The response at high frequencies can be irnproved by the use of cavity resonances on conventional lines.

The ribbon has three corrugations on each side of the aluminum armature, the corrugations having a lateral depth of about .O inch with a radius of .010 inch. Figures 1 and 2 are twice full scale, while Figure 3 is twenty-live times full size.

The pole pieces are chamfered down to very narrow lateral dimensions, ls of an inch, at their tips, this being possible because the static position of the ribbon is closely defined. The ribbon was mounted on the side of the pole pieces in an experimental model, but it will normally be cated symmetrically in the air gap, by dividing the pole pieces longitudinally and assembling the ribbon so that its edges are sandwiched between the divided pole-pieces.

As an alternative, the construction shown in Figures 4 and 5 may be employed. In these iigures, which are four times full size, the central stii area I9 of a thermo-plastic ribbon d has a cross-sectional form comprising two similar curved halves meeting at a central longitudinal apex on which is mounted a wire armature 6 of about .008 inch diameter. The ribbon is longitudinally corrugated on each side of the central stiff portion, the corrugations 5 having a preferred radius of .025 inch and overall depth of .020 inch.

The semi-circular magnet I carries iiat pole pieces 2, 3 with apertures 9 designed to give the maximum possible air path to the diaphragm without increasing the reluctance of the magnetic circuit, and so that the diaphragm will respond to frequencies below 10,000 C. P. C. as if it were freely open to the air on both sides. The pole tips l are narrowed down to very small lateral dimensions, 12- of an inch, and are very closely spaced to define a narrow air gap 8: about 0.18 inch wide, to receive the wire armature I5 of .008 inch diameter. The ribbon 4 is mounted along its longitudinal edges on spacers I3 about .015 inch thick on the pole pieces with the wire 6 positioned in the gap 3.

The wire armature II, Figure 5 is connected by copper tapes II spot-welded or otherwise fixed thereto, to terminals I2.

Alternatively the two longitudinal corrugated halves 5a, 5b of the diaphragm on either side of electrode may be in different planes and facing in opposite directions as shown in Fig. 6, so that the two halves will be located on opposite sides of the plane of the magnet poles 2, 3 with the central stili portion of the diaphragm at anV angle thereto and passing through the air gap 3 and carrying the electrode 3 therein. This has the advantage that the resonant chambers I4, I5Y

which would be designed to resonate as near the upper limit of the frequency response range as possible, or above the frequency response range. In this way the response at the resonant frequency will be the same for sound waves approaching from both the front and from the back.

It may be advantageous to have the resonant chambers I4, I5 both on the same side of the poles 2, 3 for some purposes, e. g. when the microphone is used in combination with a pressure microphone for matching purposes. In other circumstances, the arrangement shown in Fig. 6 will be advantageous.

What is claimed is:

1. A ribbon type velocity microphone comprising a structure of magnetic material having an air gap therein, a resilient ribbon having one longitudinal edge attached to the inside of said structure by a corrugated support and the other longitudinal edge attached to the outside of the structure by a similar support, said supports forming resonant cavities with the walls of said structure, said diaphgram passing through the air gap, and a linear conductor mounted on the ribbon within gap.

2. A ribbon type velocity microphone comprising a structure of magnetic material having an air gap therein, a resilient corrugated ribbon having one longitudinal edge attached to the inside of said structure by a corrugated support and the other longitudinal edge attached to the outside of the structure by a similar support, said supports forming resonant cavities with the walls of said structure, said diaphragm passing through the air gap, and a linear conductor mounted on the ribbon within the air gap.

3. A ribbon type velocity microphone comprising a structure of magnetic material having an air gap therein, a resilient diaphragm having a series of uneven longitudinal corrugations to vary the stillness thereof having one longitudinal edge attached to the inside of said structure by a corrugated support and the other longitudinal edge attached to the outside of the said structure by a similar support, said supports forming resonant cavities with the walls of said structure; said diaphragm passing through the air gap, the transverse dimension of the corrugated resilient diaphragm having a progressive variation in the longitudinal direction with respect to the width of the air gap to give progressively different resonant periods along different sections of the diaphragm, and a linear conductor mounted longitudinally on said diaphragm within said air gap.

4. A ribbon type velocity microphone accord-- ing to claim 3 wherein the said diaphragm comprises a thermoplastic material and wherein said linear conductor comprises a wire longitudinally mounted on said diaphragm and centrally positioned within the said magnetic gap.

WILLIAM DONALD CRAGG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,902,643 High Mar. 21, 1933 2,106,224 Olson Jan. 25, 1938 2,141,420 Swickard Dec. 27, 1938 2,164,157 Kennedy June 27, 1939 2,183,209 Anderson Dec. 12, 1939 FOREIGN PATENTS Number Country Date 511,034 Great Britain July 31, 1939 220,420 Great Britain Aug. 21, 1924 

